Many human diseases result from mutations that cause changes in the amino acid sequence of a protein which reduce its stability and may prevent it from folding properly. Proteins generally fold in a specific region of the cell known as the endoplasmic reticulum, or ER. The cell has quality control mechanisms that ensure that proteins are folded into their correct three-dimensional shape before they can move from the ER to the appropriate destination in the cell, a process generally referred to as protein trafficking. Misfolded proteins are often eliminated by the quality control mechanisms after initially being retained in the ER. In certain instances, misfolded proteins can accumulate in the ER before being eliminated. The retention of misfolded proteins in the ER interrupts their proper trafficking, and the resulting reduced biological activity can lead to impaired cellular function and ultimately to disease. In addition, the accumulation of misfolded proteins in the ER may lead to various types of stress on cells, which may also contribute to cellular dysfunction and disease.
Such mutations can lead to lysosomal storage disorders (LSDs), which are characterized by deficiencies of lysosomal enzymes due to mutations in the genes encoding the lysosomal enzymes. The resultant disease causes the pathologic accumulation of substrates of those enzymes, which include lipids, carbohydrates, and polysaccharides. Although there are many different mutant genotypes associated with each LSD, many of the mutations are missense mutations which can lead to the production of a less stable enzyme. These less stable enzymes are sometimes prematurely degraded by the ER-associated degradation pathway. This results in the enzyme deficiency in the lysosome, and the pathologic accumulation of substrate. Such mutant enzymes are sometimes referred to in the pertinent art as “folding mutants” or “conformational mutants.”
Fabry Disease is a LSD caused by a mutation to the GLA gene, which encodes the enzyme α-galactosidase A (α-Gal A). α-Gal A is required for glycosphingolipid metabolism. The mutation causes the substrate globotriaosylceramide (GL-3) to accumulate in various tissues and organs. Males with Fabry disease are hemizygotes because the disease genes are encoded on the X chromosome. Fabry disease is estimated to affect 1 in 40,000 and 60,000 males, and occurs less frequently in females.
There have been several approaches to treatment of Fabry disease. One approved therapy for treating Fabry disease is enzyme replacement therapy (ERT), which typically involves intravenous infusion of a purified form of the corresponding wild-type protein. Two α-Gal A products are currently available for the treatment of Fabry disease: agalsidase alfa (Replagal®, Shire Human Genetic Therapies) and agalsidase beta (Fabrazyme®; Sanofi Genzyme Corporation). ERT has several drawbacks, however. One of the main complications with ERT is rapid degradation of the infused protein, which leads to the need for numerous, costly high dose infusions. ERT has several additional caveats, such as difficulties with large-scale generation, purification, and storage of properly folded protein; obtaining glycosylated native protein; generation of an anti-protein immune response; and inability of protein to cross the blood-brain barrier to mitigate central nervous system pathologies (i.e., low bioavailability). In addition, replacement enzyme cannot penetrate the heart or kidney in sufficient amounts to reduce substrate accumulation in the renal podocytes or cardiac myocytes, which figure prominently in Fabry pathology.
Another approach to treating some enzyme deficiencies involves the use of small molecule inhibitors to reduce production of the natural substrate of deficient enzyme proteins, thereby ameliorating the pathology. This “substrate reduction” approach has been specifically described for a class of about 40 LSDs that include glycosphingolipid storage disorders. The small molecule inhibitors proposed for use as therapy are specific for inhibiting the enzymes involved in synthesis of glycolipids, reducing the amount of cellular glycolipid that needs to be broken down by the deficient enzyme.
A third approach to treating Fabry disease has been treatment with what are called pharmacological chaperones (PCs). Such PCs include small molecule inhibitors of α-Gal A, which can bind to the α-Gal A to increase the stability of both mutant enzyme and the corresponding wild type.
One problem with current treatments is difficulty in treating patients exhibiting renal impairment, which is very common in Fabry patients and progresses with disease. On average, it take between about 10-20 years for patients to decline from normal kidney function to severe renal impairment, with some countries reporting even faster declines. By some estimates, about 10% of Fabry patients receiving ERT may have moderate renal impairment. Another 25% of males and 5% of females receiving ERT have an estimated glomerular filtration rate (eGFR) of less than 30, corresponding to severe kidney impairment or even renal failure. Of these, about half have severe kidney impairment, and about half are on dialysis.
Unfortunately, renal impairment will progress despite ERT treatment. A patient having an eGFR of 30 may deteriorate to the point of needing dialysis in two to five years. About 30% of patients receiving ERT will end up on dialysis or needing a kidney transplant, depending on the start of ERT. The earlier ERT is commenced, the longer renal function may be preserved, but commencement of ERT may be delayed because Fabry disease is rare and often misdiagnosed.
Further, and as discussed above, ERT often does not sufficiently penetrate the kidneys to reduce substrate accumulation, thereby allowing further damage during disease progression. With PC treatment, the kidneys are often how the drug is cleared from the body, and renal impairment may affect drug pharmacokinetics and/or drug pharmacodynamics. Thus, there is still a need for a treatment of Fabry patients who have renal impairment.